When driving a synchronous motor, pole position information is essential and mostly using a pole position detector, pole position information is obtained. However, for the purpose of saving the mounting space of the pole position detector and reducing the cost, an art requiring no pole position detector has been developed. For example, in Japanese Patent Laid-open No. 2003-88165, to a control apparatus for a synchronous motor formed as a speed control system, the same speed instruction is input twice, and the first instruction controls the motor only by a q-axial current, and the second instruction controls the motor only by a d-axial current. An art for inferring the pole position from a ratio of the magnitude between the q-axial current and the d-axial current when the speed is fixed is disclosed. Further, the patent document describes that when a runaway is detected during the inference process of the pole position, the speed instruction is instantaneously reduced substantially to zero, and in a motor having a brake, the brake is applied.